In the float process for forming flat glass, molten glass is drawn from a melting furnace and passed to a forming chamber (or "float bath") where the molten glass is deposited onto an elongated pool of molten metal such as tin or copper or alloys thereof. There, a ribbon of glass is stretched to the desired thickness as it progresses along the elongated pool of molten metal and is then withdrawn from the forming chamber as a continuous ribbon at the exit end of the forming chamber. Because of the fluid support provided by the molten metal to the glass, glass of superior optical quality can be produced by the float process.
The roof of the float forming chamber commonly in use comprises a complex grid of relatively small ceramic pieces interlocked with one another and suspended from above by a large number of metallic rods. The design includes a large number of vertically extending electrical heating units supported within openings in the grid. Other openings in the grid are filled with blind plugs. The result is an interior roof surface which is non-planar and has a relatively large surface area and a large number of joints and vertically extending cracks and surfaces. Such a complex roof structure encourages condensation and dripping of volatilization products. The large number of crevices permits ingress of cooler exterior atmosphere which promotes condensation. The nonplanar surfaces tend to increase running and coalescing of condensation products. More recent designs of float bath roofs have simplified the support grid design so as to extend across the float chamber in only the transverse direction for the sake of simplified construction. However, the revised design still possesses the drawbacks of a large number of joints and non-planar interior surfaces.
The conventional float forming chamber entails a maze of bus bars and leads above the roof to connect the electrical heating elements to a power source. The entire electrical connection arrangement is enclosed within a large chamber known as the upper plenum, within which a controlled atmosphere is usually maintained to cool and prevent oxidation of the electrical connectors. Unfortunately, such an arrangement renders access to the heating elements and their connections extremely difficult during operation of the float chamber due to the high temperatures and closely spaced electrical conductors. It is desired occasionally to disconnect or remove a heating element to alter the heating pattern or to replace a damaged element, but because of the difficult access in prior art float chamber designs, such modifications or repairs were carried out during operation in only the most dire circumstances. Usually, the modifications or repairs were postponed until a major shut-down of the operation. Furthermore, when access was attempted during operation, a major upset of the glass forming process could result, since it was required to turn off the heat in a substantial portion of the forming chamber. Therefore, there has been a need for a more versatile heater arrangement for float forming chambers.
These problems have been alleviated considerably by the float glass forming chamber disclosed in U.S. application Ser. No. 195,283 filed on Oct. 8, 1980, by Ronald L. Schwenninger entitled "FLOAT GLASS FORMING CHAMBER WITH HORIZONTAL HEATING ELEMENTS," wherein electrical heating elements extend horizontally through side walls of the forming chamber. A variation of such a forming chamber is disclosed in U.S. patent application Ser. No. 222,396 filed on even date herewith by Earl L. May entitled "FLOAT GLASS FORMING CHAMBER WITH EXTERNALLY SUPPORTED ROOF." The horizontal heating element arrangement of these applications advantageously reduces the complexity of the roof structure by eliminating heating elements from the roof. Additionally, deploying the heating elements along side portions of the forming chamber advantageously directs heat to the portions of the forming chamber where it is most needed. However, the need arises occasionally to provide heat along the center line of the forming chamber, especially during heat-up from a cold state. Special operating situations may also render it desirable from time to time to temporarily provide center heat in the forming chamber. Therefore, in a forming chamber wherein the primary heating zones are along the sides of the forming chamber, it would be desirable to provide an arrangement for inserting temporary heating elements along the center line of the forming chamber. Such an arrangement should not revert to the complex, virtually inaccessable roof heater arrangements of the prior art, but should provide for flexibility to add or remove auxiliary heating elements during continuous operation of the forming chamber.